1. Field of the Invention
This invention relates to the field of hydrocarbon synthesis and more specifically to the field of olefin production from naphtha by steam cracking with process water from a gas-to-liquid plant, particularly from a Fischer-Tropsch synthesis process.
2. Background of the Invention
Large quantities of natural gas are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, natural gas reserves have been found in remote areas where it is uneconomical to develop the reserves due to the lack of local markets for the gas and the high cost of transporting the gas to distant markets. This high cost is often related to the extremely low temperatures needed to liquefy the highly volatile gas during transport. An alternative is to locally convert the natural gas to liquid hydrocarbon products that can be transported more cost effectively. Processes for converting light hydrocarbon gases, such as natural gas, to heavier hydrocarbon liquids are generally known in the art.
One such process, commonly known as gas to liquids (GTL) production, involves two sequential chemical transformations for converting natural gas to liquid fuels. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen and/or steam to form synthesis gas or syngas, which is a combination of carbon monoxide and hydrogen. In the second transformation employing the Fischer-Tropsch synthesis, carbon monoxide and hydrogen are converted into water and organic molecules containing mainly carbon and hydrogen. Those organic molecules containing only carbon and hydrogen are known as hydrocarbons. In addition, other organic molecules containing oxygen in addition to carbon and hydrogen, called oxygenates, may be formed during the Fischer-Tropsch synthesis. Hydrocarbons having carbons without ring formation are aliphatic hydrocarbons and may include paraffins and/or olefins. Paraffins are particularly desirable as the basis of synthetic diesel fuel.
The Fischer-Tropsch synthesis is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. A feed containing carbon monoxide and hydrogen is typically contacted with a catalyst in a reaction zone that may include one or more reactors.
The composition of a catalyst influences the relative amounts of hydrocarbons obtained from a Fischer-Tropsch catalytic process. Common catalysts for use in the Fischer-Tropsch synthesis contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, which is used throughout the present specification).
Cobalt, iron, ruthenium and/or nickel have been used as catalytic metal employed in catalysts used for the conversion of synthesis gas to hydrocarbons suitable for the production of diesel and/or gasoline fuels. Cobalt has been particularly desirable as a catalytic metal employed in Fischer-Tropsch catalysts for the production of heavy hydrocarbons from syngas. Iron has the advantage of being readily available and relatively inexpensive but has the disadvantage of a high water-gas shift activity, which converts a portion of carbon monoxide and some of the produced water to carbon dioxide and hydrogen. Nickel catalysts favor termination and are useful for aiding the selective production of methane from syngas. Ruthenium has the advantage of high activity but is quite expensive.
Typically, the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms varying from 1 to 100 or more, and thus having a range of molecular weights. Therefore, the Fischer-Tropsch products produced by conversion of natural gas commonly contain a range of hydrocarbons including gases, liquids and waxes. Depending on the molecular weight product distribution, different Fischer-Tropsch product mixtures are ideally suited to different uses. For example, Fischer-Tropsch product mixtures containing liquids may be processed to yield gasoline, as well as middle distillates. Hydrocarbon waxes may be subjected to an additional processing step for conversion to liquid and/or gaseous hydrocarbons. Thus, in the production of a Fischer-Tropsch product stream for processing to a fuel, it is desirable to maximize the production of high value liquid and/or wax hydrocarbons, such as hydrocarbons with at least 5 carbon atoms per hydrocarbon molecule (C5+ hydrocarbons).
In addition to the hydrocarbon products of the Fischer-Tropsch process, product water is formed as shown in the following equation:n CO+2n H2−>CnH2n+n H2O.The water production is quite significant because a mole of water is produced for every mole of carbon monoxide converted. For non-shifting catalysts such as employing cobalt and/or ruthenium, the water gas shift reaction is minimal so the water production approaches that of the reaction stoichiometry. For shifting catalysts such as employing iron, the water gas shift reaction is more prominent, so the overall water production is still significant but less than the reaction stoichiometry predicts. The product water from a Fischer-Tropsch reactor is typically recovered from the reactor's gas outlet, by passing the gas effluent through a separation unit, for example a condenser to generate condensate water. Alternatively, a small portion of the product water may be recovered from the reactor's liquid product stream, by employing for example a separation technique such as filtration, settling, stripping and/or centrifugation. Typically, the product water contains water-soluble organic compounds. Most of these water-soluble organic compounds are oxygenates (oxygen-containing organic compounds) such as organic acids, alcohols, aldehydes, ketones, esters, aldols, carboxylic anions, and the like. Because the product water has typically little commercial value, it is usually considered a wastewater and sent to a wastewater treatment facility for processing and removal of the dissolved organic compounds. Drawbacks to this process include high costs of building and operating the wastewater facility. Alternatively, the product water may contain a significant amount of some of these organic compounds, and their recovery from the product water may be desirable to generate by-products with commercial value.
A portion of the hydrocarbon products obtained in the Fischer-Tropsch process comprises a Fischer-Tropsch naphtha. Fischer-Tropsch naphtha can be used as a feedstock for steam cracking in the production of olefins. The steam used for such cracking typically originates from natural sources of fresh water, such as from rivers or lakes. Drawbacks to using freshwater include the cost of purchasing or processing the freshwater. Processing typically includes removing particulates.
Consequently, there is a need for reducing the costs of processing the Fischer-Tropsch product water in a wastewater treatment facility. Further needs include more efficient ways for recovering and using the Fischer-Tropsch product water. In addition, there is a need for a more economical way to produce steam for the steam cracking of Fischer-Tropsch naphtha. Additional needs include a more efficient process for producing olefins from a hydrocarbon synthesis process.